.epsilon.-Caprolactam is an important compound as an intermediate for the preparation of polyamides such as Nylon-6, and many processes are known to produce .epsilon.-caprolactam. For example, .epsilon.-caprolactam can be commercially produced by subjecting cyclohexanone oxime to the Beckmann rearrangement in the presence of an acidic medium such as fuming sulfuric acid. However, this process has drawbacks such that a large amount of ammonium sulfate, which has a less added value, is produced as a by-product.
As a process which can improve the above process, a gas phase Beckmann rearrangement using a solid catalyst is known to produce .epsilon.-caprolactam. The proposed solid catalysts used in the gas phase Beckmann rearrangement include boric acid catalysts, silica-alumina catalysts, solid phosphoric acid catalysts, complex metal oxide catalysts, zeolite catalysts, etc. Furthermore, JP-A-62-123167 and JP-A-63-54358 disclose the use of high silica metallosilicate catalysts for the production of .epsilon.-caprolactam.
Processes for producing .epsilon.-caprolactam, which is not based on the Beckmann rearrangement, are also known. For example, JP-A-2-215767 discloses a process comprising the steps of cycling methyl 6-aminocaproate to obtain .epsilon.-caprolactam, U.S. Pat. No. 5,495,016 discloses a process comprising the step of reacting 6-aminocapronitrile with water to obtain .epsilon.-caprolactam, and JP-A-9-3041 discloses a process comprising the step of reacting methyl 6-hydroxycaproate with ammonia in the presence of hydrogen and steam to obtain .epsilon.-caprolactam.
However, .epsilon.-caprolactam obtained by such processes contains various impurities. As is well known, .epsilon.-caprolactam is used as a raw material for the preparation of polyamide, and .epsilon.-caprolactam used to prepare polyamide, which is in turn used to produce synthetic fibers or films, is required to have high purity. Thus, the crude .epsilon.-caprolactam prepared by the above processes is first purified by various purification methods, and then the high purity .epsilon.-caprolactam is used to prepare polyamide for producing a product such as synthetic fibers or films.
As the purification methods, many methods are known as follows:
Distilling a crude .epsilon.-caprolactam which is obtained by rearranging cyclohexanone oxime in a sulfuric acid medium by a rectifying method (APPLIED ORGANIC CHEMISTRY, page 244 (published by TOKYO KAGAKU DOJIN in 1989);
Dissolving a crude .epsilon.-caprolactam in an organic solvent such as toluene or dimethylformamide, and crystallizing .epsilon.-caprolactam (JP-A-53-37687, JP-A-49-54389, JP-A-46-5231, etc.);
Mixing a crude .epsilon.-caprolactam with a hydrocarbon and water, separating the mixture, and extracting the .epsilon.-caprolactam with water (JP-B-36-14119, JP-A-5-294925, etc.);
Ion exchanging a crude .epsilon.-caprolactam;
Allowing a crude .epsilon.-caprolactam in contact with hydrogen at a temperature of from 100.degree. C. to 200.degree. C. in the presence of a hydrogenation catalyst (JP-A-7-109255).
However with the purification method such as distillation, crystallization, extraction or ion exchange, impurities having similar chemical properties to those of .epsilon.-caprolactam or by-products having boiling points close to that of .epsilon.-caprolactam cannot be removed sufficiently. In particular, compounds having a similar chemical structure to that of .epsilon.-caprolactam and having one carbon-carbon double bond in a molecule, such as 1,3,4,5-tetrahydroazepin-2-on, 1,5,6,7-tetrahydroazepin-2-on, and the like (hereinafter, generally referred to as "caprenolactams") cannot be removed, and thus deteriorate the quality of .epsilon.-caprolactam.
According to the observation of the researches of the present inventors, it has been found that the caprenolactams severely deteriorate the quality of .epsilon.-caprolactam. In concrete, it is found that, when an .epsilon.-caprolactam containing about 30 ppm or more of the caprenolactams is used as a raw material for producing polyamide, various problems may arise. Therefore, it is necessary to sufficiently remove the caprenolactams from the .epsilon.-caprolactam in order to obtain an .epsilon.-caprolactam having high qualities from the industrial point of view.
The present inventors have also found that the hydrogenation process is a very advantageous process since the caprenolactams are hydrogenated and converted to .epsilon.-caprolactam and a crude .epsilon.-caprolactam can be simultaneously purified, and furthermore the caprenolactams are effectively utilized. However, in the hydrogenation process, not only the caprenolactams but also by-products other than the caprenolactams participate in the hydrogenation reaction. Thus, much loading tends to be put on the hydrogenation catalyst. Therefore, the reaction efficiency decreases, and the life of the catalyst is shortened. In consequence, a crude .epsilon.-caprolactam cannot be economically treated for a long time with the hydrogenation process.
As explained above, the conventional purification methods such as distillation, crystallization, extraction, ion exchange, hydrogenation, etc. are not always satisfactory to obtain an .epsilon.-caprolactam having an industrially required purity, when the easiness of process operation and costs are taken into account.